The application of hydrophilic coatings on medical devices has become a very important method to improve biocompatibility between living tissue and the medical device. Another important property of hydrophilic coatings is to reduce the friction and to render biomedical devices slippery when wet. Medical devices like catheters, guide wires, endoscopes etc. are often sliding in direct contact with the surface of living tissue when in use. Catheters and guide wires may e.g. be introduced into the blood vessels or a catheter for catheterisation of the bladder is introduced through the urethra and withdrawn later after emptying the bladder when performing catheterisation or after some time when performing more or less permanent catheterisation. In both applications, the medical device is sliding in direct contact with a physiological surface, the walls of the blood vessels, or the mucosa of the urethra, respectively.
In order to reduce or avoid the risks of health and discomfort like inflammatory damage and degeneration caused by the medical device, hydrophilic coatings having very low friction coefficient when wet have been applied to the surface of the medical devices. Hydrophilic coatings having a low friction coefficient when wet typically comprise hydrophilic polymers such as polyvinyl pyrrolidone (PVP), polycarboxylic acids, esters, salts and amides of poly(meth)acrylic acid, copolymers of poly(methyl vinyl ether/maleic anhydride) and polyglycols like polyethyleneglycol (PEG).
According to Y. Fan (in Fan Y. L. 1990: “Hydrophilic Lubricious Coatings for Medical Applications”, Amer. Chem., Polym. Mater. Sci. Eng., 63:709-716.), the methods described in the patent literature by which hydrophilic coatings can be applied onto a substrate can be roughly divided into 5 different methods:    (1) Simple coating with hydrophilic polymers,    (2) Blending or complexing of hydrophilic polymers,    (3) Formation of interpenetrating polymeric networks,    (4) Coating with chemically reactive hydrophilic polymers and    (5) Surface grafting of hydrophilic monomers.
The first three types of hydrophilic coatings have several disadvantages: they have low abrasion resistance giving the devices a short effective lifetime. A considerable amount of polymeric residuals is released at the site where it is introduced and at the same time, this loss of polymeric material rapidly increases the friction coefficient. This abrasion or dissolution may even be so pronounced that the reduction of the friction is not effective during all of the service period of the medical device and the low friction may even have vanished when the device is to be retracted.
The fourth method involves the use of chemically reactive hydrophilic polymers that are chemically bonded to substrates or primers containing e.g. aldehyde, epoxy or isocyanate groups. The fourth coating method suffers from the drawback of the use of toxic reactive materials and in order to avoid a residual toxic effect there is a demand of long reaction times and eventually washing steps in the process. U.S. Pat. No. 4,373,009 discloses that a hydrophilic layer is formed on a substrate, e.g. wound drains, catheters, surgical tools and arterlovenous shunts, by binding unreacted isocyanate groups on the substrate surface and treating the surface with a hydrophilic copolymer made from vinyl-pyrrolidone monomers and monomers containing an active hydrogen adapted to form covalent bonds with the isocyanate.
EP 0 166 998 B1 discloses a medical instrument having a surface with a reactive functional group covalently bonded to a water-soluble polymer of a cellulose polymer, maleic anhydride polymer, polyacrylamide or a water-soluble nylon or nylon derivative and having lubricity when wetted. The substrate is treated with a solution of a compound containing the reactive functional group so that an undercoat is formed which contains this group. The undercoat is then coated with the water-soluble polymer that bonds to the functional group.
EP 0 289 996 A2 discloses a method for forming and applying a hydrophilic coating to a moulding (e.g. a razor blade), in which process a solution containing a water-soluble polymer, more particularly polyvinyl pyrrolidone or a copolymer thereof, one or more radically polymerisable vinyl monomers and a photo initiator is applied to the moulding and the applied solution is exposed to an UV radiation for curing purposes.
EP 0 991 702 A1 discloses a coating for medical devices, said coating comprising UV-radiation cross-linked hydrophilic polymer and which coating further comprises a water soluble compound like glucose, sorbitol, halides, acetates, citrates, benzoates of alkali metals or alkaline earth metals or silver, glycerine or urea.
EP 0 570 370 B1 discloses a composition for a hydrophilic coating said composition comprising a hydrophilic polymer selected from the group of polyvinyl pyrrolidone, polyvinyl pyrrolidone-polyvinyl acetate copolymer, a mixture thereof and a water-insoluble stabilizing polymer selected from the group of polymethyl vinyl ether or maleic anhydride, an ester of a copolymer, and nylon and a mixture thereof said composition being substantially more slippery when wet than when dry. Furthermore, such disclosed compositions comprise a plasticizing agent such agent preferably being selected from the group consisting of camphor, polyvinyl butyral, dibutyl phthalate, castor oil, dioctyl phthalate, acetyl tributyl citrate, dibutyl sebacate, sebacic acid and alkyl resin.
Thus, there are a number of ways to achieve hydrophilic coatings for medical devices either based on coating with two layer systems where the first layer serves as a base layer or by coating with single layer system where covalent bonding to substrate and polymer cross-linking are used for achieving coating strength. Hydrophilic coatings as described in the cited patents or patent applications tend to be brittle when dry with lack of resistance to material stresses. Cracks in the coating or horizontal debonding from the substrate are often experienced complications for such coatings. This might be overcome for non cross-linked coatings by using the teaching of EP 0 570 370 B1.
It has, however, been found that using plasticizers according to the teaching from EP 0 570 370 B1 in coatings based on hydrophilic cross-linked polymers will not result in high bonding strength and coating integrity. Neither does the use of highly hydrogen bonding, hydrophilic plasticizers like glycerine and diethylene glycol result in sufficient coating integrity nor bonding strength.
WO 90/05162 discloses an article having a two-phase surface coating of low friction when wetted. The coating consists of polyurethane and poly(N-vinyl lactam) where the poly(N-vinyl lactam) primarily forms the inner phase and the polyurethane primarily forms the outer phase. The coating is formed by application of a solution comprising the polyurethane and the poly(N-vinyl lactam) in combination with a mixture of solvents of which the poly(N-vinyl lactam) is only partly soluble in the least volatile solvent. Subsequent curing of the coating is not mentioned or indicated.
U.S. Pat. No. 5,688,855 discloses a hydrophilic coating composition comprising a mixture of a hydrogel-forming polymeric component and a polymeric water-soluble component in a solvent for dissolving both polymeric components.
Thus, there is still a need for improved hydrophilic coatings as well as for simplified procedures for the preparation of hydrophilic coatings.